The present invention relates to pressure sensor integrated circuit devices and, more particularly, to a lid for a pressure sensing device.
Integrated circuits (ICs) that can measure air pressure are useful in many applications. One such application is a tire-pressure monitoring system (TPMS). A conventional TPMS uses pressure-sensing ICs to measure the pressure of the tires of a wheeled vehicle, where each IC further includes a transmitter for transmitting sensed pressure information to a TPMS processor. The TPMS processor continuously monitors the air pressure in each of the tires and generates a signal if the pressure in any of the tires falls outside of a specified range.
FIG. 1 is a top-side perspective view of a conventional pressure-sensing device 100. FIG. 2 is a bottom-side perspective view of the device 100, and FIG. 3 is a cross-sectional side view of the device 100 along cut line Y-Y in FIG. 1. The device 100 is a quad flat no-lead (QFN) packaged device.
The device 100 comprises a die 102 attached with, e.g., a die attach material (not shown), to a metallic thermal pad 104 on the bottom of the device 100. The metallic thermal pad 104 is sometimes referred to as a die paddle. The device 100 further comprises a plurality of leads or contacts 106 and corresponding bond wires 108. The bond wires 108 connect the leads 106 to corresponding die pads (not shown) on the top surface of the die 102. In the device 100, the leads 108 do not extend out from the exterior surface of the device 100 (hence the “no-lead” part of the “quad flat no-lead” package designation for the device 100).
The top surface of the die 102 has a pressure-sensing region (not shown). A layer of flexible gel 110 covers the top of the die 102 and the bond wires 108. The gel 110 protects the die 102 and the bonding wires 304 from damage caused by the environment. The top side of the device 100 has a lid 112 with an aperture 114. The lid 112 may be metallic, or made from any other suitable material. The lid 112 typically is marked with information identifying the device 100 and its manufacturer.
The device 100 further comprises an encapsulant 116 that forms parts of side walls 118 and floor 120 of the device 100. The floor 120 is the bottom section of the device 100 that includes the paddle 104, the leads 106, and the interstitial portions of the encapsulant 116.
Between the lid 112, the side walls 118, the gel 110, and any exposed portions of the floor 120 is a cavity 122. Due to the aperture 114, the air pressure inside the cavity 122 is the same as the ambient air pressure of the proximate exterior 124 of the device 100. The aperture 114 is sized to be large enough for rapid equalization of pressure between the exterior 124 and the cavity 122 and small enough to prevent certain debris and other objects from entering the cavity 122 by way of the aperture 114 and damaging the gel 110, the die 102, and/or the bond wires 108. The pressure sensor of the die 102 is able to sense the air pressure in the cavity 122 through the flexible gel 110. The device 100 may be surface mounted, via the leads 106, to a printed circuit board (PCB) (not shown) for connection to other components of the TPMS, such as, for example, a micro controller and/or a transmitter.
Depending on a variety of factors, such as, for example, the composition of the gel 110, the gases present in the cavity 122 and the exterior 124, and the pressures of those gases—the gel 110 may suffer one or more adverse effects—such as, for example, the formation of bubbles inside the gel 110 or the stiffening of the gel 110. Accordingly, it would be advantageous to be able to better protect the gel 110.